Traction performance is one of the primary evaluation criteria for tire treads, and performance on wet surfaces such as snow and ice is an important factor in that evaluation.
Deformation of tread rubber induced by road surface asperities, rate of water drainage between the tread and road surface, and possible adhesive interactions at the interface between tread and road are some of the complex, interrelated factors that complicate the type of quantitative mechanistic understanding needed to formulate tread compounds. To further improve tire performance, those involved in tread design and manufacture continue to investigate the numerous factors that affect wet traction.
Rubber goods such as tire treads are made from elastomeric compositions that contain one or more reinforcing materials; see, e.g., The Vanderbilt Rubber Handbook, 13th ed. (1990), pp. 603-04. The first material commonly used as a filler was carbon black, which imparts good reinforcing properties and excellent wear resistance to rubber compositions. However, carbon black-containing formulations often suffer from increased rolling resistance which correlates with an increase in hysteresis and heat build-up during operation of the tire, properties which need to be minimized to increase motor vehicle fuel efficiency.
Increased hysteresis resulting from the use of carbon black can be somewhat counteracted by reducing the amount (i.e., volume) of and/or increasing the particle size of the carbon black particles, but the risks of deterioration in reinforcing properties and wear resistance limits the extent to which these routes can be pursued.
Over the last several decades, use of amorphous silica and treated variants thereof, both alone and in combination with carbon black, has grown significantly. Use of silica fillers can result in tires with reduced rolling resistance, increased traction on wet surfaces, and other enhanced properties.
In the search for further and/or additional enhancements, alternative or non-conventional fillers have been investigated. Examples include various metal hydroxides and oxides, macroscopic (e.g., 10-5000 μm mean diameter) particles of hard minerals such as CaCO3 and quartz, pumice containing SiO2, micron-scale metal sulfates, as well as clays and complex oxides.
Regardless of the type(s) of reinforcing filler(s) used in a rubber compound, enhancing dispersion of the filler(s) throughout the polymers can improve processability of the compound (rubber composition) and certain physical properties of vulcanizates made therefrom. Efforts in this regard include high temperature mixing in the presence of selectively reactive promoters, surface oxidation of compounding materials, surface grafting, and chemically modifying the polymer(s).
Chemical modification of polymers often occurs at a terminus. Terminal chemical modification can occur by reaction of a terminally active, i.e., living (i.e., anionically initiated) or pseudo-living, polymer with a functional terminating agent. Terminal modification also can be provided by means of a functional initiator, in isolation or in combination with functional termination. Functional initiators typically are organolithium compounds that additionally include other functionality, typically a nitrogen atom-containing moiety. Many functional initiators have relatively poor solubility in hydrocarbon solvents of the type commonly used in anionic polymerizations and cannot maintain propagation of living ends as well as more common alkyllithium initiators such as butyllithium; both characteristics unfortunately impact polymerization rate and efficiency negatively.
Polymers incorporating 3,4-dihydroxyphenylalanine (DOPA) have been synthesized for some time, often for adhesive applications; see, e.g., U.S. Pat. No. 4,908,404. Because these polymers can be costly and difficult to produce, so-called bulk polymers approximating their performance have been pursued. One such process is described in Westwood et al., “Simplified Polymer Mimics of Cross-Linking Adhesive Proteins,” Macromolecules 2007, 40, 3960-64, although the de-protection step employed cannot be used when the polymer contains ethylenic unsaturation. A less restrictive approach is described in U.S. patent publ. no. 2011/0286348, which provides a functional polymer that exhibits excellent interaction with various types of reinforcing fillers. The interaction between the hydroxyl group(s) of the hydroxyaryl moiety and the surface of a silica particle, while significant, probably does not involve formation of a covalent bond.
Providing additional, enhanced affinity between hydroxyaryl moieties and the silica surface via formation of a covalent bond therebetween remains desirable.